Article
Scalable Fabrication of Natural-Fiber Reinforced
Composites with Electromagnetic Interference
Shielding Properties by Incorporating Powdered
Activated Carbon
Changlei Xia 2 , Shifeng Zhang 1, *, Han Ren 3 , Sheldon Q. Shi 2, *, Hualiang Zhang 3 , Liping Cai 2
and Jianzhang Li 1
Received: 17 November 2015; Accepted: 18 December 2015; Published: 25 December 2015
Academic Editor: Kim Pickering
1
2
3
*
MOE Key Laboratory of Wooden Material Science and Application, Beijing Key Laboratory of Wood
Science and Engineering, Beijing Forestry University, Beijing 100083, China; [email protected]
Department of Mechanical and Energy Engineering, University of North Texas, Denton, TX 76203, USA;
[email protected] (C.X.); [email protected] (L.C.)
Department of Electrical Engineering, University of North Texas, Denton, TX 76203, USA;
[email protected] (H.R.); [email protected] (H.Z.)
Correspondence: [email protected] (S.Z.); [email protected] (S.Q.S.);
Tel.: +86-10-6233-6092 (S.Z.); +1-940-369-5930 (S.Q.S.); Fax: +1-940-369-8675 (S.Q.S.)
Abstract: Kenaf fiber—polyester composites incorporated with powdered activated carbon (PAC)
were prepared using the vacuum-assisted resin transfer molding (VARTM) process. The product
demonstrates the electromagnetic interference (EMI) shielding function. The kenaf fibers were
retted in a pressured reactor to remove the lignin and extractives in the fiber. The PAC was
loaded into the freshly retted fibers in water. The PAC loading effectiveness was determined
using the Brunauer-Emmett-Teller (BET) specific surface area analysis. A higher BET value was
obtained with a higher PAC loading. The transmission energies of the composites were measured
by exposing the samples to the irradiation of electromagnetic waves with a variable frequency from
8 GHz to 12 GHz. As the PAC content increased from 0% to 10.0%, 20.5% and 28.9%, the EMI
shielding effectiveness increased from 41.4% to 76.0%, 87.9% and 93.0%, respectively. Additionally,
the EMI absorption increased from 21.2% to 31.7%, 44.7% and 64.0%, respectively. The ratio of
EMI absorption/shielding of the composite at 28.9% of PAC loading was increased significantly by
37.1% as compared with the control sample. It was indicated that the incorporation of PAC into the
composites was very effective for absorbing electromagnetic waves, which resulted in a decrease in
secondary electromagnetic pollution.
Keywords: natural-fiber; composites; electromagnetic interference (EMI) shielding; activated
carbon; vacuum-assisted resin transfer molding (VARTM)
1. Introduction
Electromagnetic interferences (EMI), which are conducting and radiating electromagnetic signals
emitted by electrical circuits, perturb proper operation of surrounding electrical equipment or
cause radiative damage to living organisms [1,2]. An abundance of EMI shielding materials
and technologies have been synthesized and designed to reduce the interferences caused by
electromagnetic signals, e.g., metal sheets [3], carbon materials [4–13], electroless plating [14],
honeycomb design [1], and coating [15]. Three mechanisms of EMI shielding were investigated [16]:
reflection, which involves the application of metal sheets; absorption mechanism, which happens in
Materials 2016, 9, 10; doi:10.3390/ma9010010
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amorphous materials; and multiple reflections, which refer to the reflections at various surfaces or
interfaces in the shield.
Currently, the most common EMI shielding is metal sheets (such as nickel film [17], copper
film [18], and iron and cobalt particles [19]) because of their excellent reflection of electromagnetic
signals. However, metal sheets also have disadvantages such as high density, corrosive action,
uneconomic processing and secondary electromagnetic pollution resulting from the reflection [20].
These drawbacks deter metal sheets from practical applications in the EMI shielding field. Therefore,
a need exists to develop new materials that feature similar EMI shielding characteristics as metal
sheets but are easier to be manufactured, as well as more economical and portable.
Interest in the development of carbon-based EMI shielding has arisen driven because of the
following advantages the material offers: corrosion-resistance, low density, environmentally friendly,
and easy to manufacture. The reported EMI shielding applications of carbon materials included
carbon black [10], carbon nanotube [6,9], carbon fiber [7,8], carbon nonfiber [12], activated carbon
fiber [11], and graphene sheets [4,5]. Like other carbon materials, activated carbon demonstrates good
EMI shielding effectiveness and EMI absorption [4–12]. Activated carbon is an inexpensive resource
with low density, and the cost effectiveness and large production of activated carbon increase the
possibility of wide utilization as an EMI shielding candidate. However, no report was found in the
literature review about the use of powdered activated carbon (PAC) for EMI shielding composites.
Vacuum-assisted resin transfer molding (VARTM) process was performed to manufacture PAC
incorporated kenaf-fiber reinforced composites, which is turned out to be an excellent process for
fabricating hybrid polymer-matrix composites [21–23].
Activated carbon is a crude form of graphite with a random or amorphous structure, which is
highly porous with large internal surface area [24]. Activated carbon is a cheap resource with low
density and electromagnetic absorption properties. The low price and large production of activated
carbon increase the possibility of the wide utilization as an EMI shielding candidate. This work was
aimed at developing EMI shielding composites using kenaf fibers by incorporating PAC.
2. Materials and Methods
2.1. Materials
The kenaf bast fibers were obtained from Kengro Corporation (Charleston, MS, USA).
The sodium hydroxide (NaOH) solution (5%, w/v) was prepared using NaOH beads (ě97%,
Acros Organics, Morris Plains, NJ, USA) and deionized water. The activated carbon (12 ˆ 40 mesh)
was purchased from Calgon Carbon Corporation (Pittsburgh, PA, USA). The unsaturated
polyester AROPOL Q6585 (30% styrene, Ashland Chemicals, Roseland, NJ, USA) and tert-butyl
peroxybenzoate (t-BP, 98%, Acros Organics, Morris Plains, NJ, USA) were used to fabricate the kenaf
fiber reinforced composites.
2.2. Preparation of Powdered Activated Carbon
Using the ultra-fine pulverizing machine (RT-UF26, Rong Tsong Precision Technology Co.,
Taichung, Taiwan), the activated carbon was reduced to powder. According to the requirements
of the American National Standards Institute/American Water Works Association (ANSI/AWWA)
B600—10 standard, the particle-size distribution of PAC was: not less than 99% of the activated carbon
shall pass a No. 100 sieve, not less than 95% shall pass a No. 200 sieve, and not less than 90% shall
pass a No. 325 sieve. The particle-size distribution of the PAC was measured by a Beckman Coulter
Delsa™ Nano C Particle Analyzer (Beckman Coulter, Inc., Irving, TX, USA). PAC was dispersed
into the DI-water with a concentration of 1 mg¨ mL´1 . Prior to the particle-size measurement,
PAC/aqueous dispersion was treated by a VCX 1500 ultrasonic (Sonics & Materials Inc., Newtown,
CT, USA) for 5 min.
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2.3. Preparation of Preformed Mats
Measured by a Mettler-Toledo HB43-S Moisture Analyzer (Mettler-Toledo LLC, Columbus, OH,
USA), the average moisture content of the kenaf bast fibers was 9.1%. A mixture of 100 g kenaf fibers
and 1.8 L NaOH solution was added into a hermetical reactor (251 M, Parr Instrument Co., Moline,
IL, USA). With a saturated vapor pressure of 0.60 MPa, the alkali retting process was conducted at
160 ˝ C for one hour while the mixture was being mechanically stirred. The excessive ionic solution
was removed from the kenaf fibers by gravity after cooling to room temperature, and then by
hand-squeezing. After the retting process, the retted fibers were measured as 36.6% ˘ 1.2% of the
original ones.
PAC was stirred in 1 L water at 70 ˝ C for approximate 30 min, and then mixed with fresh alkali
retted kenaf fibers with another 30 min of stirring. The mixture was formed into a preform mat with
a dimension of approximate 100 ˆ 165 ˆ 10 mm3 , and was dried at 105 ˝ C for 24 h. Three different
amounts (10, 20, and 30 g) of PAC (6.1% moisture content, measured by a Mettler-Toledo HB43-S
Moisture Analyzer) were loaded to kenaf fibers to create three types of composites, Fiber/PAC10,
Fiber/PAC20, and Fiber/PAC30.
2.4. Composites Fabrication through VARTM Process
Four types of composites,
i.e.,
Fiber/PAC10/polyester,
Fiber/PAC20/polyester,
Fiber/PAC30/polyester and Fiber/polyester (as control panels using un-treated fibers),
were fabricated using the unsaturated polyester resin with 1.5% of t-BP catalyst and the VARTM
process, which was described in our previous reports [25]. Briefly, after applying a mold-release
agent on the surface of the mold, the preform was placed on the mold. A vacuum bag was placed
over the mold. After vacuum tubes were inserted in the bag, resin infusion was carried out by
a vacuum that was created between the mold and the bag. As a result, the catalyzed resin was
supplied to the infusion tubes. The vacuum pulled the resin along the distribution layer into the
preformed mats. A vacuum of 1.3–1.6 KPa was applied to the infusion system by the vacuum pump
(Vacmobile 20/2 System with Becker U4.20, Vacmobiles.com Limited, Auckland, New Zealand).
The resin was cured in the hot press with a pressure of 13 MPa. The resin-infused preforms were
cured in two temperature steps, i.e., 100 ˝ C for 2 h, and then 150 ˝ C for 2 h. Once the resin cured and
cooled down to room temperature, the vacuum bag and distribution layer were removed. Before
the mechanical property measurements, all the specimens were conditioned approximate 30 days to
a constant weight in a conditioning chamber maintained at a relative humidity of 50% ˘ 2% and a
temperature of 20 ˘ 3 ˝ C.
2.5. Specific Surface Area and Pore Structure Analysis
The specific surface areas of the samples were analyzed by the surface area analyzer (3Flex,
Micromeritics Instrument Corp., Norcross, GA, USA) in terms of nitrogen adsorption/desorption at
77 K. Based on the isothermal plots from P/P0 = 0.05 to 0.3, the surface areas of the samples were
obtained in accordance with the Brunauer-Emmett-Teller (BET) model. The specific pore volume
distribution of the samples was analyzed by means of the Density Functional Theory (DFT) model.
2.6. Microtopography Analysis of Fibers
The surface topologies of the un-treated kenaf fibers and Fiber/PAC30 were examined using a
Quanta 200 environmental scanning electron microscope (SEM, FEI Company, Hillsboro, OR, USA)
with an accelerating voltage of 20 kV and magnifications of 1000ˆ and 2000ˆ. Prior to the SEM tests,
the specimens were coated by a gold sputtering coater for 5 min to prevent charging of the specimen
by the SEM electron beam.
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2.7. Electromagnetic Interference Shielding Tests
The EMI shielding effectiveness measurement with both amplitude and phase properties were
carried out using the vector network analyzer (HP E8363B, Agilent Technologies, Inc., Santa Clara,
CA, USA) with a frequency ranging from 8 GHz to 12 GHz. As the most common type of
network analyzer, the vector network analyzer could be also called a gain-phase meter or an
automatic network analyzer. Two S-parameters (%), including S11 (reflected), and S21 (transmitted),
were detected and recorded. The EMI shielding effectiveness (%) and EMI absorption of composites
were plotted using with Equations (1) and (2), respectively.
EMI shielding effectiveness p%q “ p1 ´ S21q ˆ 100
(1)
EMI absorption p%q “ p1 ´ S21 ´ S11q ˆ 100
(2)
2.8. Mechanical Property
Tests
Materials 2016, 9, 10 From eachCA, USA) with a frequency ranging from 8 GHz to 12 GHz. As the most common type of network composite, twelve specimens measuring 25 ˆ 160 ˆ 3 mm3 were cut in order
analyzer, the vector network analyzer could be also called a gain‐phase meter or an automatic to examine the network modulus
of Two elasticity
(MOE)
and modulus
of rupture
(MOR) of
the composites.
analyzer. S‐parameters (%), including S11 (reflected), and S21 (transmitted), were detected and recorded. The EMI shielding effectiveness (%) and EMI absorption of composites were The Shimadzu AGS-X
universal testing machine was used for the examinations in accordance with
plotted using with Equations (1) and (2), respectively. the procedure of 3-point bending test described in ASTM D790 standard. Three-point bending set-up
(1) EMIshielding effectiveness %
1
21
100 was used with a span of 50 mm and a crosshead speed of 1.3 mm/min.
EMIabsorption %
1
21
11
100 (2) 2.9. Dynamic Mechanical Analysis
2.8. Mechanical Property Tests The dynamic mechanical
analysis
performed
a TAcut instruments
Q800 DMA
From each composite, twelve (DMA)
specimens was
measuring 25 × 160 × using
3 mm3 were in order to examine the modulus of elasticity (MOE) and modulus of rupture (MOR) of the composites. tester (TA instruments Inc., New Castle, DE, USA). Each specimen measuring about 1 ˆ 4 ˆ 30 mm3
The Shimadzu AGS‐X universal testing machine was used for the examinations in accordance with were used for the
tests. Three-point bending with a gauge length of 25.4 mm was performed.
the procedure of 3‐point bending test described in ASTM D790 standard. Three‐point bending set‐up was used with a span of 50 mm and a crosshead speed of 1.3 mm/min. These testing parameters
were conducted, including the temperature range of 22–200 ˝ C with a ramping
˝
´
1
of 5 C¨ min , and
the frequency of the oscillation at 1 Hz. The storage modulus (E1 ), loss modulus
2.9. Dynamic Mechanical Analysis (E”) and mechanicalThe dynamic mechanical analysis (DMA) was performed using a TA instruments Q800 DMA loss factor (tan(δ)) were recorded and plotted as a function of temperature.
tester (TA instruments Inc., New Castle, DE, USA). Each specimen measuring about 1 × 4 × 30 mm3 were used for the tests. Three‐point bending with a gauge length of 25.4 mm was performed. These 3. Results and Discussion
testing parameters were conducted, including the temperature range of 22–200 °C with a ramping of 5 °C∙min−1, and the frequency of the oscillation at 1 Hz. The storage modulus (Eʹ), loss modulus (Eʹʹ) 3.1. Fiber Characterization
and mechanical loss factor (tan(δ)) were recorded and plotted as a function of temperature. 3. Results and Discussion The particle-size
and distribution of PAC were determined by the light scattering method.
Figure 1a shows3.1. Fiber Characterization that the diameter of the PAC particle was mainly in 95–400 nm with a peak at
140.3 ˘ 35.6 nm. The
average particle size of PAC was calculated to be 265.9 ˘ 5.1 nm. The specific
The particle‐size and distribution of PAC were determined by the light scattering method. 2 /g
Figure 1a shows the diameter PAC particle was to
mainly 95–400 nm with peak at the micropore
surface area of the
PAC
wasthat 835.7
˘ 22.3 of mthe according
the in BET
theory,
ina which
140.3 ± 35.6 nm. The average particle size of PAC was calculated to be 265.9 ± 5.1 nm. The specific (<2 nm, as defined
by
the
International
Union
of
Pure
and
Applied
Chemistry
(IUPAC)) specific
surface area of the PAC was 835.7 ± 22.3 m2/g according to the BET theory, in which the micropore 2
surface area was(<2 708.5
mdefined /g (84.8%
of the total
surface
TheChemistry pore structure
of the PAC is shown
nm, as by the International Union of Pure area).
and Applied (IUPAC)) specific surface area was 708.5 m2/g (84.8% of the total surface area). The pore structure of the PAC is shown in Figure 1b by means
of a DFT model. The total specific pore volume was3 0.362 cm3 /g, and almost
in Figure 1b by means of a DFT model. The total specific pore volume was 0.362 cm /g, and almost all of the pore volume
was in the micropore range (Figure 1b).
all of the pore volume was in the micropore range (Figure 1b). Figure 1. Particle‐size distribution (a) and DFT pore‐size distribution; (b) of PAC. Figure 1. Particle-size
distribution (a) and DFT pore-size distribution; (b) of PAC.
4/9 Materials 2016, 9, 10
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The SEM photos of the un-treated fiber and Fiber/PAC30 are shown in Figure 2 for comparison.
PACs on the surface were visible in the Fiber/PAC30 (Figure 2b1,b2), when compared with the
un-treated fiber (Figure 2a1,a2). The PAC loading effectiveness was calculated from Equation (3)
in terms Materials 2016, 9, 10 of BET specific surface area (SA) and the results are shown in Table 1.
SAFiber{PAC ´ SAFiber
The SEM photos of the un‐treated fiber and Fiber/PAC30 are shown in Figure 2 for comparison. PAC content p%q “
ˆ 100
(3)
PACs on the surface were visible in the Fiber/PAC30 2b1,b2), when compared with the SAPAC (Figure ´ SAFiber
un‐treated fiber (Figure 2a1,a2). The PAC loading effectiveness was calculated from Equation (3) in where SA
terms of BET specific surface area (SA) and the results are shown in Table 1. PAC , SAFiber , SAFiber/PAC10 , SAFiber/PAC20 , and SAFiber/PAC30 were 835.6 ˘ 22.3, 3.3 ˘ 0.2,
115.1 ˘ 3.3, 221.0 ˘ 6.3, and 319.8 ˘ 9.5 m2 /g,SA
respectively.
SA The PAC loading effectiveness of each
/
100 PACcontent %
(3) SA
SA
Fiber/PAC was calculated by Equation (4):
where SAPAC, SAFiber, SAFiber/PAC10, SAFiber/PAC20, and SAFiber/PAC30 were 835.6 ± 22.3, 3.3 ± 0.2, 115.1 ± 3.3, PAC content ˆ Weight of Fiber{PAC
2/g, respectively. The PAC loading effectiveness of each Fiber/PAC was 221.0 ± 6.3, and 319.8 ± 9.5 m
PAC loading effectiveness
p%q “
ˆ 100
PAC feed{ p1 ` PAC moisture contentq
calculated by Equation (4): PAC content
Weight of Fiber/PAC
(4)
where PAC contentPACloadingeffectiveness
was calculated by Equation
(3), and PAC moisture content100 was 6.1%.
%
(4) The PAC
PAC feed⁄ 1 PAC moisture content
loading effectiveness was found to be 66.6%, 77.9% and 84.7% for Fiber/PAC10, Fiber/PAC20,
where PAC content was calculated by Equation (3), and PAC moisture content was 6.1%. The PAC and Fiber/PAC30, respectively. These results show that the loading effectiveness increases with the
loading effectiveness was found to be 66.6%, 77.9% and 84.7% for Fiber/PAC10, Fiber/PAC20, and increment
of PAC feed.
The reason
that
hydrophobic
PAC effectiveness was absorbed
onto with the hydrophilic
Fiber/PAC30, respectively. These was
results show that the loading increases the fiber surface
that resulted from the very big specific surface area and high porosity as well as
increment of PAC feed. The reason was that hydrophobic PAC was absorbed onto the hydrophilic fiber surface that resulted from the very big specific hydrophobic
surface area and high porosity as well PAC
as when
absorbability.
The surface
of the
Fiber/PAC
became
and
absorbed
more
the PAC absorbability. The surface of the Fiber/PAC became hydrophobic and absorbed more PAC when the loading further increased, which resulted in more effective absorption for PAC. The results
PAC loading further increased, which resulted in more effective absorption for PAC. The results reflect as the increment of the PAC loading effectiveness.
reflect as the increment of the PAC loading effectiveness. Figure 2. SEM observation of the un‐treated fiber (a1,a2); and Fiber/PAC30 (b1,b2). Figure 2. SEM observation of the un-treated fiber (a1,a2); and Fiber/PAC30 (b1,b2).
Table 1. Contents of PAC loaded fibers. Specimen Table
1. Contents of PAC loaded fibers.
PAC Feed PAC Loading Effectiveness Content (%) (g) (%)
PAC Feed
PAC Loading – Effectiveness
Fiber – Specimen
(g)10 (%)
Fiber/PAC10 66.6 Fiber/PAC20 77.9 Fiber
–20 –
Fiber/PAC30 84.7 Fiber/PAC10
1030 66.6
Fiber/PAC20
20a PAC = powdered activated carbon. 77.9
Fiber/PAC30
30
84.7
a
PAC = powdered5/9 activated carbon.
PAC a Fiber Content
(%)
0.0 100.0 a
PAC
Fiber
13.4 86.6 26.2 73.8 0.0
100.0
38.0 62.0 13.4
86.6
26.2
73.8
38.0
62.0
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3.2. Composites Properties
The physical properties of composites manufactured through VARTM technology are shown
in Table 2. The composite density was decreased with the increment in PAC content because
of the porosity
increment of the composites. The BET specific surface areas of Fiber/polyester,
Materials 2016, 9, 10 Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester were 0.1, 0.4, 5.0,
and 5.5 m23.2. Composites Properties /g, respectively, suggesting the increment of the porosity of the composites.
The physical properties of composites manufactured through VARTM technology are shown in Table 2. The composite density was 2.decreased the composites.
increment in PAC content because of the Table
Contentswith of the
porosity increment of the composites. The BET specific surface areas of Fiber/polyester, Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester were 0.1, 0.4, 5.0, and 5.5 Density
Thickness
Content (%)
Composite
m2/g, respectively, suggesting the increment of the porosity of the composites. (Kg¨ m´3 )
Fiber/polyester
Fiber/PAC10/polyester
Composite Fiber/PAC20/polyester
Fiber/polyester Fiber/PAC30/polyester
a
(mm)
Fiber
Resin c
PAC b
Table 2. Contents of the composites. 1159.4 (44.5) a
2.6 (0.2)
66.7
0.0
33.3
Density
Thickness
Content (%) 1157.3
(15.1)
3.5 (0.1)
64.2
10.0
25.8
−3)
b Resin c (Kg∙m
(mm)
1064.7
(20.4)
4.2
(0.1) Fiber
57.9 PAC 20.5
21.7
1159.4 (44.5) 1036.2
(18.8) a 2.6 (0.2) 4.8 (0.2) 66.7 47.1 0.0 28.9 33.3 24.1
Fiber/PAC10/polyester 1157.3 (15.1) 3.5 (0.1) 64.2 10.0 25.8 mean (standard deviation); PAC b = powdered activated carbon; c polyester was used here.
Fiber/PAC20/polyester 1064.7 (20.4) 4.2 (0.1) 57.9 20.5 21.7 Fiber/PAC30/polyester 1036.2 (18.8) 4.8 (0.2) 47.1 28.9 24.1 a mean (standard deviation); PAC b = powdered activated carbon; c polyester was used here. From the 3-point
bending test results
shown in Figure
3, the mechanical properties
(MOE and MOR)
of the composites decreased with the increase in PAC content, showing the
From the 3‐point bending test results shown in Figure 3, the mechanical properties (MOE and MOR) of the composites decreased with the increase in PAC content, showing the same variation same variation
trend as the composite density results. MOE (3.3 GPa) and MOR (33.2 MPa)
trend as the composite decreased
density results. MOE and
(3.3 51.4%,
GPa) and (33.2 MPa) of the of the Fiber/PAC30/polyester
by 52.0%
as MOR compared
with Fiber/polyester
Fiber/PAC30/polyester decreased by 52.0% and 51.4%, as compared with Fiber/polyester (6.9 GPa (6.9 GPa MOE and 68.3 MPa MOR). The main reason could be that the increment in porosity reduced
MOE and 68.3 MPa MOR). The main reason could be that the increment in porosity reduced the the mechanical
property [26].
mechanical property [26]. Figure 3. Modulus of elasticity (a) and rupture (b) of Fiber/polyester (C1), Fiber/PAC10/polyester Modulus of elasticity (a) and rupture (b) of Fiber/polyester (C1), Fiber/PAC10/polyester
(C2), Fiber/PAC20/polyester (C3), and Fiber/PAC30/polyester (C4) composites. Figure 3.
(C2), Fiber/PAC20/polyester (C3), and Fiber/PAC30/polyester (C4) composites.
Storage modulus is a crucial index for measuring the energy storage capability of material after elastic deformation, which was an index of resilience. Figure 4a shows the Eʹ of the four composites. Storage
modulus is a crucial index for measuring the energy storage capability of material
In general, the Eʹ of the four composites showed the maximum values around 32–39 °C, and then after elastic
deformation,
which was
an index
ofof resilience.
Figure
4a shows
the
E1 of the four
decreased as the temperature increased because the chain mobility increasing of the polymer matrix [27]. Similar to the mechanical properties (MOE and MOR), after adding PAC, Eʹ decreased composites.
In general, the E1 of the four composites showed the maximum values around 32–39 ˝ C,
compared with Fiber/polyester, indicating that of
additional PAC mobility
reduced the elastic and then considerably decreased as
the temperature
increased
because
the chain
increasing
of the
properties of the composites. Figure 4b represents the loss modulus of the four composites. The Eʹʹ polymer matrix
[27].
Similar
to
the
mechanical
properties
(MOE
and
MOR),
after
adding
PAC,
of Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester composites followed 1
E decreased
considerably compared with Fiber/polyester, indicating that additional PAC reduced
the same trend as that of Eʹ. However, the Eʹʹ of Fiber/polyester showed a lower value as compared the elastic with that of Fiber/PAC10/polyester composite because of the additional high‐porous PAC. Figure 4c properties of the composites. Figure 4b represents the loss modulus of the four composites.
The E” of shows the loss factor of the four composites, reflecting the damping of molecular movement within Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester composites
the material. As the tan(δ) increased, the flow of the viscous molecules became harder, and the energy followed the
same trend as that of E1 . However, the E” of Fiber/polyester showed a lower value
increasingly dissipated. With the increase of additional PAC, the tan(δ) increased gradually, as compared
with that of Fiber/PAC10/polyester composite because of the additional high-porous
indicating that PAC was able to restrict the movement of the chains and thus increase the damping PAC. Figure 4c shows the loss factor of the four composites, reflecting the damping of molecular
6/9 movement within the material. As the tan(δ) increased, the flow of the viscous molecules became
harder, and the energy increasingly dissipated. With the increase of additional PAC, the tan(δ)
increased gradually, indicating that PAC was able to restrict the movement of the chains and thus
Materials 2016, 9, 10
Materials 2016, 9, 10 7 of 9
effect. Additionally, as shown in Figure 4c, the glass transition temperatures of the Fiber/polyester, increase
the damping effect. Additionally, as shown in Figure 4c, the glass transition temperatures of
Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester composites were the Fiber/polyester, Fiber/PAC10/polyester,
Fiber/PAC20/polyester,
and Fiber/PAC30/polyester
obtained to be 113.4 °C, 83.2 °C, 57.6 °C, and 56.3 °C, respectively, which showed a increment with composites
were obtained to be 113.4 ˝ C, 83.2 ˝ C, 57.6 ˝ C, and 56.3 ˝ C, respectively, which showed a
the amount of added PAC. Materials 2016, 9, 10 increment
with the amount of added PAC.
effect. Additionally, as shown in Figure 4c, the glass transition temperatures of the Fiber/polyester, Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester composites were obtained to be 113.4 °C, 83.2 °C, 57.6 °C, and 56.3 °C, respectively, which showed a increment with the amount of added PAC. Figure 4. DMA results of the four composites, including storage modulus (a); loss modulus (b); and Figure
4. DMA results of the four composites, including storage modulus (a); loss modulus (b);
damping parameter (c). and
damping parameter (c).
3.3. EMI Shielding Figure 4. DMA results of the four composites, including storage modulus (a); loss modulus (b); and 3.3.
EMI Shielding
damping parameter (c). Figure 5 shows the EMI shielding and absorption of the four types of composites. The averaged Figure
5 shows the EMI shielding and absorption of the four types of composites. The averaged
EMI properties at the
the range EMI shielding of 3.3. EMI Shielding EMI properties
at
range of of 8–12 8–12 GHz GHz are are summarized summarized in in Table Table 3. 3. The The
EMI
shielding
Fiber/PAC30/polyester was was
93.0%, which the of
Fiber/PAC30/polyester
93.0%,
whichwas wasincreased increasedby by 124.6% 124.6% compared compared with with the
Figure 5 shows the EMI shielding and absorption of the four types of composites. The averaged Fiber/polyester composite. Since PAC is different from the common EMI shielding mechanism of Fiber/polyester
composite.
Sinceof PAC
isGHz different
from the common
shielding
mechanism
of
EMI properties at the range 8–12 are summarized in Table EMI
3. The EMI shielding of reflection, it prefers the absorption of the EMI signals. The EMI absorptions of Fiber/polyester, Fiber/PAC30/polyester was 93.0%, which increased by 124.6% compared with the reflection,
it prefers the absorption
of the
EMIwas signals.
The EMI
absorptions
of Fiber/polyester,
Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester were 31.7%, Fiber/polyester composite. Since PAC is different from the common EMI shielding mechanism Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester were 21.2%, 21.2%,of 31.7%,
reflection, it prefers the absorption of the EMI signals. The EMI absorptions of Fiber/polyester, 44.7%, and 64.0%, respectively. The EMI absorption of Fiber/PAC30/polyester was increased by 44.7%, and 64.0%, respectively. The EMI absorption of Fiber/PAC30/polyester was increased by
Fiber/PAC10/polyester, Fiber/PAC20/polyester, and Fiber/PAC30/polyester were 21.2%, 31.7%, 201.7%, as
as compared
compared with
with that
that ofof Fiber/polyester.
Fiber/polyester. The in the
the EMI
EMI 201.7%,
The percentages percentages of of absorption absorption in
44.7%, and ratios 64.0%, of respectively. The EMI absorption of Fiber/PAC30/polyester was increased by shielding and EMI absorption/shielding are presented in Table 3. The ratios of EMI shielding
andas ratios
of EMI
absorption/shielding
presented
in of Table
3. The
of EMI
201.7%, compared with that of Fiber/polyester. are
The percentages absorption in ratios
the EMI absorption/shielding increased from 41.8%
41.8% to
to 50.9%
50.9% and
and 68.7%
68.7% for
for the
the Fiber/PAC10/polyester
Fiber/PAC10/polyester to
to absorption/shielding
increased
from
shielding and ratios of EMI absorption/shielding are presented in Table 3. The ratios of EMI Fiber/PAC20/polyester and Fiber/PAC30/polyester, which indicated that the absorption percentage Fiber/PAC20/polyester
Fiber/PAC30/polyester,
indicated
that
the absorption percentage
absorption/shielding and
increased from 41.8% to 50.9% which
and 68.7% for the Fiber/PAC10/polyester to in EMI shielding increased with the increment of the PAC content. The EMI absorption/shielding of in EMIFiber/PAC20/polyester and Fiber/PAC30/polyester, which indicated that the absorption percentage shielding increased with the increment of the PAC content. The EMI absorption/shielding
Fiber/PAC30/polyester (68.7%) was significantly increased by 37.1% (ANOVA test, α = 0.001, p‐value in EMI shielding increased with the increment of the PAC content. The EMI absorption/shielding of of
Fiber/PAC30/polyester
(68.7%) was significantly increased by 37.1% (ANOVA test, α = 0.001,
−79), compared with that of Fiber/polyester (50.1%), which indicated that the percentage of Fiber/PAC30/polyester (68.7%) was significantly increased by 37.1% (ANOVA test, α = 0.001, p‐value = 9.07 × 10
´
79
p-value = 9.07 ˆ 10 ), compared with that of Fiber/polyester (50.1%), which indicated that the
−79), compared with that of Fiber/polyester (50.1%), which indicated that the percentage of = 9.07 × 10
reflection in shielding was shielding
significantly The increase absorption in the percentage
ofEMI reflection
in EMI
wasreduced. significantly
reduced. of The
increaseresulted of absorption
reflection in EMI shielding was significantly reduced. The increase of absorption resulted in the decrease of the secondary electromagnetic pollution. resulted
in the decrease of the secondary electromagnetic pollution.
decrease of the secondary electromagnetic pollution. Figure 5. EMI shielding effectiveness (a) and absorption (b) of the four composites. Figure 5. EMI shielding effectiveness (a) and absorption (b) of the four composites.
Figure 5. EMI shielding effectiveness (a) and absorption (b) of the four composites. 7/9 7/9 Materials 2016, 9, 10
8 of 9
Table 3. EMI shielding and absorption of the four composites.
Composite
Fiber/PAC10/polyester
Fiber/PAC20/polyester
Fiber/PAC30/polyester
Fiber/polyester
Increment (%) c
EMI Shielding
EMI Absorption a
EMI Absorption/Shielding
(%)
(%)
(%)
31.7 (4.2)
44.7 (4.4)
64.0 (3.7)
21.2 (6.5)
201.7
41.8 (5.7)
50.9 (5.4)
68.7 (4.1)
50.1 (10.3)
37.1
b
76.0 (2.6)
87.9 (1.8)
93.0 (1.1)
41.4 (5.5)
124.6
a EMI = electromagnetic interference; b mean (standard deviation) which were calculated from the data in
8–12 GHz; c increments were calculated from the data of Fiber/PAC30/polyester and Fiber/polyester.
4. Conclusions
The composites were manufactured from kenaf fibers, PAC and polyester using the VARTM
technology. The PAC loading effectiveness was determined using the BET specific surface area
analysis, which showed an increase with the increase in PAC loading. EMI shielding tests were
carried out with a variable frequency ranging from 8 GHz to 12 GHz. As the PAC content increased
from 0% to 28.9%, the EMI shielding and absorption increased from 41.4% to 93.0%, and 21.2%
to 64.0%, respectively, and the ratio of EMI absorption/shielding significantly increased by 37.1%.
The results showed that PAC was more effective at EMI signal absorption rather than reflection,
which benefited the decrease of secondary electromagnetic pollution. However, the mechanical and
dynamic mechanical properties decreased after the addition of PAC. Further work is planned to
improve these properties of the composites.
Acknowledgments: This research was supported by the Special Fund for Forestry Research in the Public
Interest (Project 201504502), National Science Foundation (NSF) CMMI 1247008, Fundamental Research
Funds for the Central Universities (NO. TD2011-12), Beijing Natural Science Foundation (Project 2151003),
and National Natural Science Foundation of China (Project 31000268/C160302). We thank Angela Nelson
(College of Engineering, University of North Texas) for proof reading.
Author Contributions: Changlei Xia, Shifeng Zhang and Liping Cai performed the experiments. Han Ren
and Hualiang Zhang conducted the EMI tests. Changlei Xia, Shifeng Zhang, Sheldon Q. Shi, Liping Cai and
Jianzhang Li wrote and revised the manuscript. All authors contributed to the analysis of the data and approved
the final version of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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